1) Firearms get sold or stolen...a stolen weapon could be used in a crime.
Yes, and then some of our socialist paradise states can then enact laws making the registered owner of the firearm used to perpetrate a crime civilly liable for not ‘Controlling the Weapon’ or some other such nonsense.
4) There may be some question about use of firearm changing the "imprint"---metal changes from use and wear.
It can also be easily modified to give a different pattern, but anyone with machinist skills. This was driven home to me as a teenager, when a fellow trade school student, built his own 0.22 revolver with only the basic machine tools in his fathers basement. The damn thing worked pretty good too, I can just imagine what someone with 20 years experience as a die maker could produce.
You mean like locking it up, or keeping it in a safe?